The invention relates to a wind tunnel having a free jet test section in which the direction of flow can be rotated during the operation of the wind tunnel. According to the teachings of the invention, this is possible while the wind is blowing, without having to tolerate substantial losses of aerodynamic quality of the flow with regard to uniformity of the velocity distribution and magnitude of the degree of turbulence.
A wind tunnel in which the direction of flow can be rotated during the operation of the wind tunnel is already known from German Patent Document DE 3715016. This wind tunnel is principally intended for aeroacoustic investigations of motor vehicles and is distinguished in that the fan is designed to simulate the flow noise of the motor vehicle standing in an acoustic chamber. However, what is to be understood from the design for the simulation of circumcirculation noise is not evident from the teachings of the described arrangement. In the disclosed arrangement, furthermore, a device is described in which the exit cross-section of the free jet nozzle is markedly smaller than the cross-sectional area of the motor vehicle against which flow is directed. It emerges from this that no excessively stringent requirements are imposed upon the quality of the aerodynamic circumcirculation simulation. For gust simulation, the nozzle can, in accordance with the teachings of the invention DE 3715016, be rotated in the manner of a pendulum about its vertical axis. The connection between nozzle and nozzle prechamber takes place in this case by means of a flexible hose conduit. Since in the course of the rotation of the nozzle the curvature of this hose conduit and consequently also the velocity profile of the elbow flow becoming established constantly changes, the uniformity of the velocity distribution in the nozzle exit cross-section which is attainable by this device is rather moderate.
A further wind tunnel in which the direction of the air flow on emerging from a rigid free jet nozzle can be altered is known from German Patent Document DE 40 07 230. In this case, airfoil wing profiles which are disposed in the form of a cascade and are pivotable in each instance about their vertical axis are used for the purpose of changing the direction of flow. In order that the change of direction of the free jet should be possible, this cascade with the pivotably disposed profiles is mounted behind the exit of the wind tunnel nozzle. The object to be investigated in the wind tunnel test section, which according to the teachings of the described arrangement is preferably a motor vehicle or a motor vehicle model, is therefore exposed to the jet flow influenced by the cascade. This jet flow is characterized by the wakes behind the profiles, which wakes are unavoidable even with the most careful design of the profiles. The wind tunnel could scarcely be suitable for aeroacoustic investigations for example, since upon the free jet noise there is also in addition superposed the noise of the profile cascade which is circumcirculated at high velocity, namely the outflow velocity from the nozzle. With regard to the jet turbulence as well, such a wind tunnel device does not satisfy the requirements to be satisfied for the execution of precise aerodynamic investigations, since the jet turbulence is greatly influenced by the wakes behind the individual profiles and this turbulence moreover still alters to a large extent with the passage distance of the flow as a result of the mixing of these wakes with the flow passing through between the profiles. This would mean for example that the turbulence at the front part of a motor vehicle to be investigated has a different structure from that at the rear.
A similar device for generating gust in a wind tunnel is described by W. Baumert et al. of the Deutsche Forschungs- und Versuchsanstalt fur Luft- und Raumfahrt in the report DFVLR Reports 89/05 in paragraph 7.4. However, in this case what are involved are relatively small angle changes which take place with relatively high frequency, e.g. gust amplitude .+-.2.degree. at a frequency of 20 Hz. This provides a device which is essentially suitable for flight-mechanics and especially aeroelastic investigations and is unsuitable for measurements on motor vehicles, where substantially greater angular deviations of up to 20.degree. are required. Another disadvantage of this device is further, as is evident from FIG. 7.4-3 of said publication, that the gust angle alters very greatly over the passage distance of the flow, while for example at a spacing of 900 mm a still approximately constant angle of approximately 0.7.degree. becomes established over the entire height of the wind tunnel nozzle, and already after a further 900 mm passage distance the gust angle at a gust frequency of 5 Hz is twice as great at the upper jet margin than at the lower. Thus, this device is also suitable at all events for indicative studies and does not satisfy the stringent requirements for development work in the sector of motor vehicle aerodynamics and especially the flow acoustics of motor vehicles.
A further wind tunnel, in which an acceptable simulation of the circumcirculation can be achieved with a relatively small nozzle exit cross-section, is known from U.S. Pat. No. 5,025,659. The improved flow simulation with a relatively small nozzle exit cross-section is achieved by lengthening the nozzle by means of slotted walls. These slotted walls permit a pressure compensation and prevent the development, which is unavoidable in the case of a pure free jet, of a mixing zone, the transverse dimension of which increases in the direction of flow, i.e. along the test specimen. However, this process functions only in the case of a symmetric or approximately symmetric flow onto the test specimen. Since moreover nozzle and sidewalls are rigid, the wind tunnel according to the U.S. Patent is unsuitable for gust simulation.
Details concerning an aeroacoustic wind tunnel are known for example from the AGARD Report No. 601 "Problems in Wind Tunnel Testing Techniques" of 1973 and especially from the paper included in this report by T. A. Holbeche and J. Williams "Acoustic Considerations for Noise Experiments at Model Scale in Subsonic Wind Tunnels". The test section is surrounded by a so-called anechoic chamber with high acoustic absorption. Both on the inlet side and also on the outlet side in relation to the fan which drives the air flow through the test chamber in the circuit, the flow guidance system includes acoustic attenuators which, in the case of a wind tunnel with a closed flow guidance system, are expediently combined with the deflection angles. In this case, the insertion absorption of these acoustic attenuators is to be of such magnitude that the sound pressure level originating from the fan, in the test chamber, is sufficiently, e.g. 10 dB, below the sound pressure level which is generated by the test jet. In these circumstances, the fan noise in the test chamber is no longer perceptible and also no longer has a disturbing effect on accurate acoustic measurements.
However, the wind tunnel design known from this publication cannot be used for gust simulation, since no measures are provided for the instantaneous alteration of the direction of flow.
Accordingly, the object of the invention is to develop further the known wind tunnel designs and to provide a wind tunnel which, besides a simulation of the complete circumcirculation which is sufficiently good even for development work, permits a possibility of gust simulation by rotation of the wind direction and in addition, with an appropriate design of the wind tunnel, also permits aeroacoustic investigations.
Such a wind tunnel is of advantage in all cases where rapid changes of the direction of flow, as occur for example in the natural wind, are to be simulated, or where the intention is to study the aerodynamic properties and the reaction of a circumcirculated body in a flow field with a rapidly varying direction of flow. This is of particular importance in the sector of vehicle aerodynamics, for the investigation of side wind effects. At the present time, it is only possible to investigate vehicles for their side wind suitability by rotating the vehicle mounted on a rotatable platform with its longitudinal axis against the direction of flow. The aerodynamic data measured under these conditions, such as for example air-generated forces and air-generated moments of forces, are however applicable only to the case of steady flow, which in the practical operation of vehicles is substantially less important than the non-steady process which takes place when a vehicle enters a side wind gust. Attempts are made to record this effect by side wind installations past which vehicles travel at a specified speed. However, such investigations may be made, when developing a vehicle, only at stages of development which are already very far advanced, demand a high expenditure and are in addition still, as they must take place on a test track, highly dependent on the weather and feasible only under particular meteorological conditions. Moreover, when using a side wind test installation it is only possible to record effects which concern the guidance of the vehicle and not for example other influences which are essential for the vehicle development, such as for example the alteration of the flow noise upon a rapid alteration of the incident flow direction. Specifically in the case of luxury vehicles, which have only a low noise level in the interior, the alteration of the noise due to circumcirculation when the effective wind direction varies is very disturbing. These changes may not be recorded in conventional aeroacoustic wind tunnels for vehicle aerodynamics, in which wind tunnels the incident forced flow direction can be altered only by rotation of the vehicle mounted on a rotary platform. Thus, in order to solve this problem, it is necessary to have recourse to costly tests while the vehicle is being driven. However, these tests while the vehicle is being driven can take place only when a prototype is already available, i.e. substantial changes in development are scarcely possible any longer. Such a wind tunnel with adequate acoustic properties, i.e. a sufficiently low noise level, and at the same time an incident flow direction which can be altered in operation, can also be used in the quality control of high-quality cars. In fact, within the context of the final inspection, it is possible to check the interior noises with an appropriate wind speed and rotation of the nozzle, in order to detect causes of disturbing flow noises, such as for example incompletely sealed apertures, incorrectly adjusted doors, etc. In this way, it is possible to save considerable workshop service and warranty services.
Accordingly, the object is to provide a wind tunnel with which the effects of the variable oblique flow incidence can be simulated in a manner which is sufficiently true to reality.
Such a device is provided by the present invention of a wind tunnel having a free jet test section and having a wind tunnel nozzle which is rotatable when the wind tunnel is operating in order to generate a flow of variable direction. This nozzle comprises a nozzle front part which is rotatable about a vertical axis and which is separated from the upstream nozzle rear part in the region of the turning point of the nozzle contour between the--viewed from the nozzle center line--concave curvature of the nozzle rear part and the convex curvature of the nozzle front part and at this separation location is surrounded by an inlet rounding. This wind tunnel can additionally be equipped with acoustically effective devices which are integrated into the flow guidance system, in order to reduce the sound pressure level to such an extent that the perception and metrological recording of the flow noises which are dependent upon the incident flow direction is also possible.